A speaker is known that generates sound on the basis of a digital signal (for example, see Patent Literature 1). The digital speaker can achieve high sound quality due to a lack of deterioration of sound quality by an analog system from audio amps and the like during transmission to the speaker. Further, for small-sized equipment such as mobile phones, the use of a digital terminal is preferred from the standpoint of equipment design due to the digital terminal being smaller than an analog terminal (so-called pin jack), and thus digital speakers, which generate sound on the basis of a digital signal output from the digital terminal, are increasingly important.
A digital speaker requires an array of separate sound generating devices, a device for each bit of the inputted digital signal. However, due to speaker units using a permanent magnet and voice coil often being utilized conventionally as each of the sound generating devices, a problem occurs due to mutual induction between coils. Further, differences between the individual coils also cause a problem of decreased sound quality. Also miniaturization is difficult due to the requirement that the number of speaker units matches the bit count.
Further, in the conventional speaker unit, the voice coil is provided at the center of a cone, and directionality is relatively high due to the generation of sound pressure by piston-like vibration of the cone. Thus the conventional speaker unit is inherently inappropriate for lowering directionality and providing sound over a wide angle.
Further, Patent Literature 2 discloses a digital speaker in which the number of electrodes arranged on one piezoelectric element is the same as the bit count. Either the voltage applied to each electrode differs for the corresponding bit, or the surface area of each of the electrodes corresponds to the bit. However, Patent Literature 2 does not disclose a circuit applying a voltage to each of the electrodes, and enablement cannot be realized using the disclosed configuration. In particular, how voltage is applied to a central portion of the piezoelectric element is unclear. Further, the voltage of each bit is applied separately to the central portion and circumferential portion of the piezoelectric element, and thus frequency characteristics of each bit in the piezoelectric element are not uniform.